Fiber optic distributed sensing overcomes the inherent limitations of traditional technologies, such as motion detectors, cameras, thermocouples and strain gauges, enabling monitoring solutions with new advantages for the protection of people and critical assets, especially when monitoring in inaccessible or inhospitable environments.
In a distributed sensor, the whole optical cable is the sensor itself. Typically, the fiber is integrated around (or into) a valuable asset (building, pipeline, cables, etc.). A single optical fiber can replace thousands of traditional single-point sensors, providing a significant reduction in installation, calibration, and maintenance costs. In addition, assets can now be monitored where previously this was impractical due to their size, complexity, location or environment.
The monitored structure in the present invention is any object or area under observation. The monitored structure can be the area enclosed into the sensor system completely or partially. The monitored structure can be an object, such as pipe, cable, fence, wall, etc. that is integrated into the system by placing the optical fibers in a proximity to the object or integrated into the object, for example installed into, wrapped by or winded around the object. A single monitored structure can be comprised of different elements that are objects of, generally, different types, for example, monitored areas and monitored objects at the same time.
The inherent distributed sensing nature of fiber optic sensors can be used to create unique forms of sensors for which, in general, there may be no counterpart based on conventional sensor technologies. Optical fiber is cheap, light, pliable, and immune to electromagnetic interference (EMI), which makes it a cost-effective, flexible and an inert sensor medium.
Most of the existing distributed sensors are based on Distributed Scattering Sensing (Raman or Brillion), which has a limited sensitivity and immunity to various noises.
With modern low loss fibers and solid state laser diode sources, it has become possible to develop systems having sensing fibers up to several tens of kilometers in length. In one field of application, these fibers can, for example, be placed under ground, carpets, imbedded in walls, under roads, or under turf. In such installations the sensors can be effective in the detection of personnel, vehicles, animals, etc. into a protected area of interest.
There is a need in well concealed fiber optic sensing systems to provide economic means for location of events, recognition and identification of disturbance events, and positive means for periodically and remotely proof testing the integrity of the sensing fiber.
A few distributed optical fiber sensing systems based on a Sagnac interferometer and a Mach-Zehnder interferometers have been previously developed. These systems depend on the interferometric detection of phase differences between two optical signals whose relative phases have been shifted by changes in the optical properties of their respective paths caused by an external factor, such as, for example, acoustical or mechanical perturbation. The change in optical property of the fiber path may be in the form of elongation, change in index of refraction, change in birefringence, or a combination of these or related effects.
While interferometer-based sensor systems have been developed with a number of refinements, such systems have not been fully optimized for use in applications that require simultaneous monitoring of different types of objects.
Prior art, for example, U.S. Pat. No. 6,621,947 and No. 6,778,717, both by E. E. Tapanes et al., discloses a principle (device and method) behind the counter-propagating signals detection in distributed fiber sensor. Optical signals are launched from a single light source into the waveguide (fence) and simultaneously detected by a two separate photo-detectors. The difference between registered signals allows detecting the place of the optical fence intrusion. Any parameter that alters the fiber will affect both counter-propagating signals in a similar fashion. Thus, the U.S. Pat. No. 7,519,242 by E. E. Tapanes shows the particular buried fiber configuration that is specifically suited for detecting an intruder walking across the ground beneath which fibers are burried.
However, it is desirable to have a distributed sensing system where the response to the potential perturbation can vary along the cable, being adjusted to either the real system environment or a particular system application. In other words, the real system implementation might require different sensitivity at different areas being monitored, depending on, for example, different structure layouts, various perturbation probabilities within different areas, or the different nature of perturbations within different areas. The prior art does not provide such desirable functionality.
Polarization phase shift variations are caused by dynamically varying changes in a signals polarization state versus the principal polarization axis of the interferometer. As a result, received counter-propagating signals can potentially interfere constructively or destructively. Thus, it is also desirable, in addition, to have a distributed sensing system where polarization effects are intrinsically managed by the system to dynamically address the variation of polarization states along the fiber sensor. Polarization management techniques for distributed fiber sensing have been disclosed in U.S. Pat. No. 7,499,176 by A. R. Adams and U.S. Pat. No. 7,499,177 by J. Katsifolis, as well as in U.S. Pat. No. 7,142,736 by J. S. Patel et al. Moreover, the U.S. Pat. No. 7,139,476 discloses the method of disturbance event detection/location by using the changes in the states of polarization of counter propagating signals themselves. However, in all these configurations, the external polarization management was applied to the system with uniform perturbation response across the perimeter, as mentioned above.
There is a need for a distributed fiber sensing link with non-uniform perturbation response across the perimeter, which would drastically expand the applicability of the system.
Although existing security application of distributed sensor systems are valuable in detecting events over large areas, they are not always sufficiently sensitive, capable of dynamic adjustment/control, convenient in implementation, properly camouflaged or economical for determining the location of events of different nature at different areas. Thus, several systems would be required to distinguish between perturbations, caused, for example, by events of different nature and/or perturbations that correspond to different environments encountered by a system installation.
Operational pipelines are subject to complex, highly non-linear temporal and spatial processes that usually make it difficult to differentiate between faults and stochastic system behaviors. This makes detecting failures/intrusions a challenging task, leading to integrating different types of data that is remotely captured from several sources, such as proposed fiber-optic system, as well as pressure transient signals and flow (velocity) information. The various types of (high frequency) data can be time synchronized. There is a need for a system capable of detecting small problems that might be precursors of catastrophic bursts, also enabling prompt detection and localization of larger leaks and malfunctioning equipment such as valves.
There is also a need for a technology to be used in earthquake continuous monitoring/early warning system. For example, when the system has detected a wave (P-wave-representing the warning of a future imminent major earthquake), the visual and acoustic quake alarm can be started.